Solubility of lignin in microemulsion system
However, the solubilization effect is one of the most important features of the microemulsion system, therefore the solubilities of lignin in different microemulsions were measured and listed in Table 1. The lignin sample originated from bagasse is insoluble in both water and octane (Table 1, entries 1 and 2), but it can dissolve in n -propanol with the solubility of 19.9 g L-1 (Table 1, entry 3). Basing on the above constructed SFME system and the recognition of its subregion, the solubilities of lignin in different microemulsions were investigated. At first, different O/W microemulsions at a fixed RP/Ovalue of 8.16 were selected to test the effect of water content (Table 1, entries 4-8), considering that water is a kind of green and easily available solvent. The solubility of lignin in then -propanol/octane system without water (Table 1, entry 4, point a in Figure 3) is close to that in n -propanol. However, due to the solubilization effect and the changing polarity of microemulsions, the solubility of lignin is enhanced significantly by adding water to then -propanol/octane system. For example, when the WW value increases from 7.6 to 15.7 wt. %, the solubility of lignin sharply rises from 28.6 to 79.1 g L-1 (Table 1, entries 5 and 6, points b and c in Figure 3), which is compatible to that obtained in ionic liquids.37,38 And it remains about 71.9 g L-1 when the WW value further increases to 31.2 wt. % (Table 1, entries 7 and 8, points d and e in Figure 3). It implies that suitable polarity for the microemulsion may act as an important parameter to affect the solubility of lignin, because the polarity of the microemulsion increases with the elevated WW value as illustrated in Figure S2A, while lots of polar groups in lignin macromolecules also provide contribution to the solubilization. In order to give a more understanding about the solubilization effect in all subregions, the solubility of lignin affected by n -propanol content (WP) was further determined under the fixed mass ratio of water to octane (RW/O) basing on the maximum value obtained above (Table 1, entries 6, 9 and 10, points c, f and g in Figure 3). The solubility of lignin sharply increases at first and then decreases slightly with the increase of WP, and it reaches the maximum value at the WP value of 75.1 wt. %, which is the same point as mentioned above. Besides, it is worth noticing that the solubilities of lignin in the most given O/W microemulsions improve greatly, compared with those in B.C and W/O microemulsions (Table 1, entries 9 and 11, points f and j in Figure 3). More interestingly, all the solubilities of lignin in the given microemulsions are larger than that in n -propanol or n -propanol/octane binary system under the same conditions (Table 1, entries 5-11 vs 3 and 4), this does verify enhanced solubilization of the microemulsions. Therefore, the microemulsion possesses the best solubility for lignin is regarded largely with the composition of 15.7 wt. % water, 9.2wt. % octane and 75.1 wt. % n -propanol, exhibiting about four times of the lignin solubility in comparison to that inn -propanol.
Table 1 . Solubility of lignin in different microemulsions